The invention relates to an improved process for recovering an alkali metal terephthalate and an alkyene glycol from poly(alkylene terephthalate) waste from used products, for example packaging, films, fibres or bottles, such recovered polyterephthalates possibly also containing other impurities such as fillers, additives, colorants, or being mixed with other solid materials such as paper labels, polyethylene and/or polypropylene closures (for bottles), etc.. The terephthalic acid which can be produced from such polyterephthalate solutions, in particular by precipitation using sulphuric acid, is normally for the production of new poly(alkylene terephthalate)s intended for the production, for example, of new packaging for use in the food industry.
Such processes have already been described in the technical literature. Those which the invention is intended to improve employ alkaline saponification of such poly(alkylene terephthalate)s, usually poly(ethylene terephthalate)s (PET), such saponification leading as a result to the production of an alkali metal polyterephthalate and the corresponding alkylene glycol.
The oldest patents relating to such saponification processes include British patent GB 822 834 in which the saponification reaction is carried out by reacting poly(alkylene terephthalate)s with a relatively dilute solution of an alkali metal hydroxide, in particular sodium hydroxide (an 18% sodium hydroxide solution according to Example 1), the reaction conditions being adjusted so as to encourage maximum salting out of the alkali metal polyterephthalate formed in the reaction solution. Because of the relative solubility of sodium terephthalate, the authors of that patent recommended increasing the salting out capacity of the sodium terephthalate by adding an alcohol, or substituting the aqueous solution with a mixture of an insoluble alkali metal hydroxide and an alcohol.
In French patents 9101025/2672049 and 9213583/2697839, the alkali metal (or alkaline-earth metal) hydroxide is employed in the solid state or in the presence of a quantity of water at least equal in weight to that of the alkali or alkaline-earth metal hydroxide, or in the molten state in the absence of water, the reaction being carried out at a temperature of more than 120xc2x0 C. and preferably in the range 140xc2x0 C. to 180xc2x0 C. The advantage of those processes resides in the possibility of carrying them out continuously.
The alkali metal terephthalate solutions resulting from said saponification operations are usually coloured and mixed with other dissolved matter; hence substantial purification of such solutions has to be carried out before treating them, in particular by acidification, to obtain a terephthalic acid of satisfactory purity.
A further difficulty which until now has not been overcome satisfactorily is recovery of most of the polyol, in particular polyethylene glycol liberated by the saponification reaction. In a solution proposed to that effect in International patent application PCT WO-A-95/29952, the saponification reaction is carried out at a temperature which is higher than the polyol evaporation temperature, in particular over 200xc2x0 C. The polyol can then be recovered by condensing. In that application, the quality of the alkaline polycarboxylate produced, in particular the sodium terephthalate, is not affected by such high temperatures even if an oxygen-depleted atmosphere is used to control impurity carbonisation reactions at such high temperatures.
The problem of coloration and the amount of other matter dissolved in the alkali metal polycarboxylate solution is not solved by that process. Clearly, the terephthalic acid finally obtained by acid precipitation from the sodium terephthalate solutions obtained cannot be of the desired purity for fresh production of polyethylene glycol polyterephthalate, for example for use for food packaging.
The aim of the invention is to further improve saponification processes of the type described, in particular to optimise the reaction conditions, to obtain the polycarboxylates more directly, in particular alkali metal polyterephthalates with a purity which is compatible with the demands of the food industry, while employing conditions which demand less energy.
A further aim of the invention is to ensure that the reactions are as complete as possible, such that all of the other principal reactants, in particular the polyols liberated during the saponification operation, are almost completely recovered.